20620041 electrical-motors
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Electric MotorsElectric Motors
Electric MotorsElectric Motors
Electric MotorsElectric Motors
Introduction
Types of electric motors
Assessment of electric motors
Energy efficiency opportunities
Selecting Electric Motors
IntroductionIntroduction
What is an Electric Motor?
• Electromechanical device that converts electrical energy to mechanical energy
• Mechanical energy used to e.g.• Rotate pump impeller, fan, blower
• Drive compressors
• Lift materials
• Motors in industry: 70% of electrical load
IntroductionIntroduction
How Does an Electric Motor Work?
How Does an Electric Motor Work?
IntroductionIntroduction
IntroductionIntroduction
How Does an Electric Motor Work?
IntroductionIntroduction
Three types of Motor Load
Motor loads Description Examples
Constant torque loads
Output power varies but torque is constant
Conveyors, rotary kilns, constant-displacement pumps
Variable torque loads
Torque varies with square of operation speed
Centrifugal pumps, fans
Constant power loads
Torque changes inversely with speed
Machine tools
Type of Electric MotorsType of Electric Motors
Classification of Motors
Electric Motors
Alternating Current (AC) Motors
Direct Current (DC) Motors
Synchronous Induction
Three-PhaseSingle-Phase
Self ExcitedSeparately Excited
Series ShuntCompound
Type of Electric MotorsType of Electric Motors
DC Motors – Components
• Field pole• North pole and south pole
• Receive electricity to formmagnetic field
• Armature• Cylinder between the poles
• Electromagnet when current goes through
• Linked to drive shaft to drive the load
• Commutator• Overturns current direction in armature
Type of Electric MotorsType of Electric Motors
DC motors
• Speed control without impact power supply quality
• Changing armature voltage
• Changing field current
• Restricted use
• Few low/medium speed applications
• Clean, non-hazardous areas
• Expensive compared to AC motors
Type of Electric MotorsType of Electric Motors
• Electrical current reverses direction
• Two parts: stator and rotor• Stator: stationary electrical component
• Rotor: rotates the motor shaft
• Speed difficult to control
• Two types• Synchronous motor
• Induction motor
AC Motors
Type of Electric MotorsType of Electric Motors
• Constant speed fixed by system frequency
• DC for excitation and low starting torque: suited for low load applications
• Can improve power factor: suited for high electricity use systems
• Synchronous speed (Ns):
AC Motors – Synchronous motor
Ns = 120 f / PF = supply frequencyP = number of poles
Type of Electric MotorsType of Electric Motors
AC Motors – Induction motor
• Most common motors in industry
• Advantages:
• Simple design
• Inexpensive
• High power to weight ratio
• Easy to maintain
• Direct connection to AC power source
Type of Electric MotorsType of Electric Motors
AC Motors – Induction motor
Components
• Rotor
• Squirrel cage: conducting barsin parallel slots
• Wound rotor: 3-phase, double-layer, distributed winding
• Stator• Stampings with slots to carry 3-phase windings
• Wound for definite number of poles
(Automated Buildings)
Type of Electric MotorsType of Electric Motors
How induction motors work• Electricity supplied to stator
• Magnetic field generated that moves around rotor
• Current induced in rotor
AC Motors – Induction motor
Electromagnetics
Stator
Rotor
• Rotor produces second magnetic field that opposes stator magnetic field
• Rotor begins to rotate
Type of Electric MotorsType of Electric Motors
AC Motors – Induction motor
• Single-phase induction motor• One stator winding
• Single-phase power supply
• Squirrel cage rotor
• Require device to start motor
• 3 to 4 HP applications
• Household appliances: fans, washing machines, dryers
Type of Electric MotorsType of Electric Motors
AC Motors – Induction motor
• Three-phase induction motor• Three-phase supply produces magnetic
field
• Squirrel cage or wound rotor
• Self-starting
• High power capabilities
• 1/3 to hundreds HP applications: pumps, compressors, conveyor belts, grinders
• 70% of motors in industry!
Type of Electric MotorsType of Electric Motors
Speed and slip
• Motor never runs at synchronous speed but lower “base speed”
• Difference is “slip”
• Install slip ring to avoid this
• Calculate % slip:
AC Motors – Induction motor
% Slip = Ns – Nb x 100 Ns
Ns = synchronous speed in RPMNb = base speed in RPM
Assessment of Electric MotorsAssessment of Electric Motors
Efficiency of Electric Motors
Motors loose energy when serving a load
• Fixed loss
• Rotor loss
• Stator loss
• Friction and rewinding
• Stray load loss
Assessment of Electric MotorsAssessment of Electric Motors
Efficiency of Electric Motors
Factors that influence efficiency• Age• Capacity• Speed• Type• Temperature• Rewinding• Load
Assessment of Electric MotorsAssessment of Electric Motors
Efficiency of Electric Motors
Motor part load efficiency• Designed for 50-100% load
• Most efficient at 75% load
• Rapid drop below 50% load
Assessment of Electric MotorsAssessment of Electric Motors
• Motor load is indicator of efficiency
• Equation to determine load:
Motor Load
Load = Pi x HP x 0.7457
= Motor operating efficiency in %HP = Nameplate rated horse powerLoad = Output power as a % of rated powerPi = Three phase power in kW
Assessment of Electric MotorsAssessment of Electric Motors
Motor Load
Three methods for individual motors
• Input power measurement• Ratio input power and rate power at 100%
loading
• Line current measurement• Compare measured amperage with rated
amperage
• Slip method• Compare slip at operation with slip at full
load
Assessment of Electric MotorsAssessment of Electric Motors
Motor Load
Input power measurement
• Three steps for three-phase motors
Step 1. Determine the input power:
Pi = Three Phase power in kWV = RMS Voltage, mean line to
line of 3 PhasesI = RMS Current, mean of 3 phasesPF = Power factor as Decimal
1000
3xPFxIxVPi
Assessment of Electric MotorsAssessment of Electric Motors
Motor Load
Input power measurementStep 2. Determine the rated power:
Step 3. Determine the percentage load:
rr xhpP
7457.0
%100xP
PiLoad
r
Load = Output Power as a % of Rated PowerPi = Measured Three Phase power in kWPr = Input Power at Full Rated load in kW
Pr = Input Power at Full Rated load in kWhp = Name plate Rated Horse Powerr = Efficiency at Full Rated Load
Assessment of Electric MotorsAssessment of Electric Motors
Motor Load
Result
1. Significantly oversized and under loaded
2. Moderately oversized and under loaded
3. Properly sized but standard efficiency
Action→ Replace with more efficient,
properly sized models
→ Replace with more efficient, properly sized models when they fail
→ Replace most of these with energy-efficient models when they fail
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
1. Use energy efficient motors
2. Reduce under-loading (and avoid over-sized motors)
3. Size to variable load
4. Improve power quality
5. Rewinding
6. Power factor correction by capacitors
7. Improve maintenance
8. Speed control of induction motor
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
• Reduce intrinsic motor losses
• Efficiency 3-7% higher
• Wide range of ratings
• More expensive but rapid payback
• Best to replace whenexisting motors fail
Use Energy Efficient Motors
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
Use Energy Efficient Motors
Power Loss Area Efficiency Improvement
1. Fixed loss (iron) Use of thinner gauge, lower loss core steel reduces eddy current losses. Longer core adds more steel to the design, which reduces losses due to lower operating flux densities.
2. Stator I2R Use of more copper & larger conductors increases cross sectional area of stator windings. This lower resistance (R) of the windings & reduces losses due to current flow (I)
3 Rotor I2R Use of larger rotor conductor bars increases size of cross section, lowering conductor resistance (R) & losses due to current flow (I)
4 Friction & Winding Use of low loss fan design reduces losses due to air movement
5. Stray Load Loss Use of optimized design & strict quality control procedures minimizes stray load losses
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
2. Reduce Under-loading
• Reasons for under-loading• Large safety factor when selecting motor
• Under-utilization of equipment
• Maintain outputs at desired level even at low input voltages
• High starting torque is required
• Consequences of under-loading• Increased motor losses
• Reduced motor efficiency
• Reduced power factor
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
2. Reduce Under-loading
• Replace with smaller motor• If motor operates at <50%
• Not if motor operates at 60-70%
• Operate in star mode• If motors consistently operate at <40%
• Inexpensive and effective
• Motor electrically downsized by wire reconfiguration
• Motor speed and voltage reduction but unchanged performance
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
3. Sizing to Variable Load
• Motor selection based on
• Highest anticipated load: expensive and risk of under-loading
• Slightly lower than highest load: occasional overloading for short periods
• But avoid risk of overheating due to
• Extreme load changes
• Frequent / long periods of overloading
• Inability of motor to cool down
X
Motors have ‘service factor’ of 15% above
rated load
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
4. Improve Power Quality
Motor performance affected by
• Poor power quality: too high fluctuations in voltage and frequency
• Voltage unbalance: unequal voltages to three phases of motor
Example 1 Example 2 Example 3
Voltage unbalance (%) 0.30 2.30 5.40
Unbalance in current (%) 0.4 17.7 40.0
Temperature increase (oC)
0 30 40
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
4. Improve Power Quality
Keep voltage unbalance within 1%
• Balance single phase loads equally among three phases
• Segregate single phase loads and feed them into separate line/transformer
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
5. Rewinding
• Rewinding: sometimes 50% of motors
• Can reduce motor efficiency
• Maintain efficiency after rewinding by
• Using qualified/certified firm
• Maintain original motor design
• Replace 40HP, >15 year old motors instead of rewinding
• Buy new motor if costs are less than 50-65% of rewinding costs
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
6. Improve Power Factor (PF)
• Use capacitors for induction motors
• Benefits of improved PF
• Reduced kVA
• Reduced losses
• Improved voltage regulation
• Increased efficiency of plant electrical system
• Capacitor size not >90% of no-load kVAR of motor
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
7. Maintenance
Checklist to maintain motor efficiency
• Inspect motors regularly for wear, dirt/dust
• Checking motor loads for over/under loading
• Lubricate appropriately
• Check alignment of motor and equipment
• Ensure supply wiring and terminal box and properly sized and installed
• Provide adequate ventilation
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
8. Speed Control of Induction Motor
• Multi-speed motors
• Limited speed control: 2 – 4 fixed speeds
• Wound rotor motor drives
• Specifically constructed motor
• Variable resistors to control torque performance
• >300 HP most common
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
8. Speed Control of Induction Motor
• Variable speed drives (VSDs)
• Also called inverters
• Several kW to 750 kW
• Change speed of induction motors
• Can be installed in existing system
• Reduce electricity by >50% in fans and pumps
• Convert 50Hz incoming power to variable frequency and voltage: change speed
• Three types
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
8. Speed Control of Induction Motor
Direct Current Drives
• Oldest form of electrical speed control
• Consists of
• DC motor: field windings and armature
• Controller: regulates DC voltage to armature that controls motor speed
• Tacho-generator: gives feedback signal to controlled
Selecting Electric Motors
• How much power is needed
• How much electrical power is available
• Do you have enough capacity in service entrance panel (breaker box)
What Size Motor to Select
Selecting Electric Motors
Power Supply
• Single Phase, 115 or 230 volts– limited to 7 1/2 hp– most farms and homes– many motors will run on 115 or 230 volts
Selecting Electric Motors
Power Supply
• 3-Phase, 208, 230 or more volts– 4 wires in power line– up to 1,000 hp– little or no light flickering– cost less– last longer– pay extra to install 3-phase power lines
Selecting Electric Motors
Service Entrance Capacity
• SEP must have about 3 times more amperage capacity than the amp rating on the nameplate of the motor– for extra amps for starting the motor– if motor is 20 amps, SEP must be at least 60
amps
• May need a separate SEP
Selecting Electric Motors
What Motor Speed to Select
• Determine speed of equipment
• Speed is in RPM’s
• Most common: 1750
• If different speed is needed, use pulley, gear, or chains to convert
Selecting Electric Motors
Motor Duty
• Motor Duty = amount of time the motor is operating under full load, and how much time it is stopped
• Continuous Duty: constant full load for over 60 minutes at a time
• Intermittent Duty: fully loaded for 5, 15, 30, or 60 minutes
Selecting Electric Motors
Starting Loads
• Easy Starting Loads: – Shaded Pole Induction– Split Phase– Permanent-Split, Capacitor-Induction– Soft-Start
Selecting Electric Motors
Starting Loads
• Difficult Starting Loads– Capacitor-Start, Induction-Run– Repulsion-Start, Induction-Run– Capacitor-Start, Capacitor-Run– Three-Phase, General-Purpose– Perkey Concept: use tractor PTO to start– Repulsion-Start, Capacitor-Run
Selecting Electric Motors
Other Factors to Consider
• Direction of Rotation
• Cost
• Maintenance– motors with brushes cause radio interference– repulsion-start interferes at starting– motors with brushes require more maintenance
Bearing Types
• Sleeve Bearings: brass, bronze or tin lined cylinder
• Ball Bearings: round steel balls surround the shaft in a special cage
Mounting Position
• Sleeve Bearings: parallel to floor– may need to rotate end shield to prevent oil from
running out of reservoir
• Ball Bearing: any position
Mounting Base
• Rigid (fixed to frame)
• Rigid (adjustable screws)
• Sliding Rails
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
THANK YOUTHANK YOU
FOR YOUR ATTENTIONFOR YOUR ATTENTION
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